The present invention relates to a coating apparatus for coating a magnetic coating material onto a substrate so as to make a magnetic recording medium such as a magnetic tape, and more particularly, to a gravure coating apparatus capable of eliminating any unevenness of coating of the magnetic paint with respect to the substrate.
According to a conventional coating apparatus for coating a magnetic coating material onto a substrate available for a coating type magnetic tape, a predetermined amount of the magnetic coating material is sprayed onto the substrate by a nozzle or a feeder bar. Alternatively, there has been another coating apparatus using a gravure roll and a back-up roll. In the latter method, the coating material accumulated in an ink pan is captured onto engraved surface portions of the gravure roll and is transferred onto the tape substrate which is running through a space defined between the gravure roll and the back up roll.
However, according to such conventional coating apparatus, air remains at the engraved portions of the gravure roll if high speed coating is performed, so that a sufficient amount of the coating material cannot be retained within the engraved portions, or air is mixed into the coating material. As a result, uniform coating over the substrate is not attainable.
Further, even if an ordinary coating at a constant speed is performed, adhesion unevenness of the magnetic coating material to the gravure roll may occur due to variation in the viscosity of the coating material, to thereby disadvantageously cause coating unevenness onto the substrate. To avoid this drawback, the inventor has previously proposed a coating apparatus in which magnetic force is applied to peripheral surfaces of the gravure roll and the back up roll as disclosed in Japanese Patent Application No 59-247370 and now Patent No. 61-126630. According to this proposal, magnets are accommodated in both a cup roll (gravure roll) and the back up roll, so that magnetic force is given to the respective peripheral surfaces of these rolls. Further, with respect to the cup roll, first and second magnets are disposed at a position adjacent to a contact portion relative to the substrate and a position interior of a contact portion relative to a magnetic recording liquid (magnetic coating material) accumulated in a pan, respectively. With this structure, non-uniform adhesion of the magnetic coating material to the cup roll has been overcome to some extent.
However, according to the coating apparatus, there has been inherent disadvantage in that a shape of a liquid accumulating portion defined at a contact portion between the cup roll and the substrate is still unstable due to imbalance between the increasing rotational speed of the cup roll and the viscosity of the coating material. Due to such unstable shape of the liquid mass at the contact portion, coatIng uniformity over the substrate would be degraded.